1. Field of the Invention
The present invention relates to a method for inhibiting electromigration-induced phase segregation. More particularly, the present invention relates to a method for inhibiting electromigration-induced phase segregation in solder joints.
2. Description of Related Art
Since the development of modern electronic devices advances lightness, slimness, shortness, smallness and multi-function, the demands of number of solder joints as well as the carried electric current in high performance devices are both increased to meet the above requirements. Moreover, the sizes of solder joints are also required to reduce, so as to improve the communication efficiency between devices, and to increase a plenty of input/output terminals in the same packing volume. Under such the scenario that reducing joints sizes accompanying with increasing the carried electric current, the current densities applied in solder joints hence are pushed to a level, in which electromigration are of concern. Electromigration refers to a mass transport phenomenon, which the thermally activated atoms/ions within a conductor would migrate in the direction as the electron flow due to momentum transfer between atoms/ions and electrons through the scattering. Such the mass flow would cause the mass depletion near the cathode and accumulation near the anode, leading to failures in conducting lines. In multi-phase alloys, electromigration becomes much more complicated, mainly responsible for the variation in the mobility of diffusion species. Such as in the eutectic PbSn (63 wt. % Sn-37 wt. % Pb) solder joints, the dominated diffusion species are known to be the Pb when the operation temperatures over 100° C., which would be driven toward anode more quickly. Since the solder is constrained by the solder/pad interfaces at the two ends, electromigration of Pb would be obstructed at the anode end of solder where the Pb accumulated as a layer. The occupancies of the Pb at the anode induces a back flow of Sn toward the cathode, resulting in the segregation of Pb-rich and Sn-rich phases as a two-layer structure. Such the electromigration-induced phase segregation seriously deteriorates the microstructure of eutectic solders, which has disturbed the electronic industry in recent years.
FIG. 1 is a schematic diagram illustrating solder joints electrically connected between a chip and a wiring board according to a conventional technique. Referring to FIG. 1, the solder joint 110 is electrically connected between a chip 120 and a wiring board 130. In the conventional technique, since the size of the solder joint 110 is decreased and the current flowing through the solder joint 110 is increased, the current density applied into the solder joint 110 is then increased significantly. Therefore, the electromigration behaviours trends to be obvious in the solder joint 110.
More particularly, when a flowing direction of a current I is from the wiring board 130 toward the chip 120 via the solder joint 110, it represents a flowing direction of an electron flow E is from the chip 120 toward the wiring board 130 via the solder joint 110. Now, thermally activated atoms (not shown) within the solder joint 110 are affected by the electron flow E, and thus the electromigration behaviours are likely to occur in the solder joint 110. A cross-sectional view of an as-reflow Cu/eutectic PbSn/Cu solder joint is shown in FIG. 2 (A). As can be clearly seen, the interwoven lamellar microstructure composed of a dark phase 140 and a white phase 150 reveals a typical eutectic structure of Pb—Sn alloys after a reflow. The dark phase 140 and white phase 150 are Pb-rich and Sn-rich respectively. At the interfaces of solder 160/Cu 170, the chemical reaction(s) between solder 160 and Cu 170 produced an equivalent reaction product(s) or intermetallic compound(s) (IMC) layer 180 at the both side. FIG. 2 (B) shows the same solder joint that had been imposed a current density of 104 A/cm2 for 7 days. The direction of the electron flow was from Cu 170 at the cathode side C to Cu 170 at the anode side A. Several changes in the microstructures compared to FIG. 2 (A) should be noted herein: (1) Pb had segregated from the interwoven lamellar structure of eutectic PbSn as a dense Pb-rich layer at the anode side A of solder 160. Next to the Pb-rich layer in solder 160, a layer of Sn-rich formed. (2) The morphologies of the IMC layers 180 were dissymmetry. The IMC layer 180 of the cathode C revealed irregularly in comparison with the one of the anode side A. (3) The depletion of Cu 170 metal of the cathode C is in a higher speed than that occurred in the anode A. The above three changes were known to be the electromigration behaviours in solder joints, which has disturbed the electronic industry in recent years.